For some years, in particular, tyre manufacturers have been making particularly great efforts to develop novel solutions to a problem dating from the very beginning of the use of wheels fitted with tyres of inflated type, namely how to allow the vehicle to continue its journey despite a significant or complete loss of pressure from one or more tyres. For decades, the spare wheel was regarded as the sole and universal solution. Then, more recently, the considerable advantages related to its possible removal have become apparent. The concept of “extended mobility” was developed. The associated techniques make it possible to run with the same tyre, according to certain limits to be observed, after a puncture or a fall in pressure. This makes it possible, for example, to drive to a breakdown point without having to halt, under often hazardous conditions, to fit the spare wheel.
Self-sealing compositions capable of making it possible to achieve such an objective, by definition capable of automatically ensuring, that is to say without any external intervention, the airtightness of a tyre in the event of perforation of the latter by a foreign body, such as a nail, are particularly difficult to develop.
In order to be able to be used, a self-sealing layer has to satisfy numerous conditions of a physical and chemical nature. In particular, it has to be effective over a very wide range of operating temperatures, this being the case over the whole of the lifetime of the tyres. It has to be capable of sealing the hole when the perforating article remains in place; when the latter is expelled, it has to be able to fill in the hole and to render the tyre airtight.
Numerous solutions have been devised but have not been able truly to be deployed in vehicle tyres, in particular for lack of stability over time or of effectiveness under extreme operating temperature conditions, or also due to difficulties in manufacturing and/or using these self-sealing compositions.
Thus, in order to contribute to maintaining good high-temperature effectiveness, the document U.S. Pat. No. 4,113,799 (or FR-A-2 318 042) provided, as self-sealing layer, a composition comprising a combination of butyl rubbers having high and low molecular weights which are partly crosslinked, optionally in the presence of a small portion of thermoplastic styrene elastomer.
The document U.S. Pat. No. 4,228,839 provided, as tyre self-sealing layer, a rubber mixture comprising a first polymer material which decomposes when irradiated, such as polyisobutylene, and a second polymer material which crosslinks when irradiated, preferably a butyl rubber.
The document U.S. Pat. No. 4,426,468 has for its part also provided a tyre self-sealing composition based on crosslinked butyl rubber having a very high molecular weight.
A known disadvantage of butyl rubbers is that they exhibit significant hysteresis losses (i.e., a high tan δ level) over a broad temperature spectrum, a disadvantage which has repercussions on the self-sealing compositions themselves with a strong increase in the hysteresis and significant penalization of the rolling resistance of the tyres.
Self-sealing compositions based on an unsaturated diene elastomer (natural rubber) have also been described for the same type of application, in particular in U.S. Pat. No. 4,913,209, U.S. Pat. No. 5,085,942 and U.S. Pat. No. 5,295,525.
These compositions are characterized by the combined presence of a high content of hydrocarbon resin as tackifying agent (“tackifier”), always greater than 100 parts by weight per 100 parts of solid elastomer, and of a large amount of elastomer (isoprene) in the liquid state. In point of fact, such a high resin content, apart from the fact that its incorporation requires a very long kneading of the elastomer matrix, can also be harmful to the hysteresis and consequently to the rolling resistance of the tyres. In addition, a large amount of liquid elastomer gives a high fluidity to the composition, which is the source of other disadvantages, in particular of a risk of creep of the self-sealing composition during use at relatively high temperature (typically greater than 60° C.) frequently encountered during the use of some tyres.
Furthermore, the documents WO2010/009849 and WO2010/012413 of the Applicant Companies describe a self-sealing composition comprising natural rubber (NR), a hydrocarbon resin, a very low content of carbon black and optionally a liquid plasticizer.